The present invention relates to scanner mechanisms providing both high speed and high resolution. More specifically the invention is directed to high precision scanners used in the art of integrated circuit manufacture and inspection.
In order to provide a high resolution scanner (on the order of nanometer resolution) it is important to know the location of the scanner head at all times. To provide both high speed and high resolution the scanner must traverse the inspection zone in a minimum amount of time, stop, and return in the opposite direction for the next scan.
The production of integrated circuits is accomplished by photolithographic processes on wafer substrates and entails precise alignment of exposure masks. Precise alignment is difficult to accomplish in environments susceptible to vibration. In addition to the manufacturing steps involved, inspection of the wafer is also necessary both during manufacturing and following completion of the process. Again, inspection entails precise alignment of the inspection scanner. In order to exposure or inspect a wafer there must be relative motion between the operating mechanism and the wafer and this may be accomplished by holding the scan head stationary and moving the wafer or by holding the wafer stationary and moving the head, or both. In either case motion is required and motion induces vibration.
Due to the reduction of scale in modern IC manufacturing processes, even small vibrations may render a chip useless. As components shrink, the dimensions become sufficiently small that misalignment may occur if the resolution of the system is not in the nanometer range. Small vibrations may “shake” the apparatus so that the device is rendered useless below a specific resolution. As component dimensions shrink, the problem becomes more severe.
Vibration may be dealt with in one of two ways. Either vibration is allowed and vibration suppression is used or active vibration cancellation may be employed. Vibration suppression entails the use of dampeners (shock absorbers, rubber mounts, high mass bases, etc.). Vibration cancellation entails determining the characteristics of a vibration source and generating a secondary source of equal and opposite magnitude thereby canceling the vibration and not requiring isolation in the first place.
FIG. 1 is a schematic representation of the concept of active vibration cancellation as implemented in various prior references. A carriage 10 is driven by an actuator 12 to move through a desired scanning motion relative to a fixed base 14. Carriage 10 has a mass M1 and moves with a velocity v1 along a predefined line of motion. Acceleration of carriage 10 is achieved by forces provided by actuator 12 which, according to Newtonian laws of motion, result in equal but opposite reaction forces transmitted via the actuator to base 14. In order to cancel vibrations caused by these reaction forces, the system includes a countermass 16 which is driven by a countermass actuator 18 to move along a path parallel to the line of motion of carriage 10. If countermass actuator 18 is made to generate forces equal to, but opposite to those generated by actuator 12, the reaction forces on base 14 are also equal but opposite, and cancel out.
It should be noted that the active vibration cancellation described here inherently causes oscillating stress patterns in the base, thereby rendering it virtually impossible to completely eliminate vibrations. Furthermore, although theoretically effective to cancel much vibration, the arrangement of FIG. 1 is far from simple to implement. Specifically, when working at very high resolutions, effective balancing of forces generated by two actuators becomes increasingly difficult. Any imbalance in forces or imperfect synchronization of the forces will result in net forces on the base which will cause vibrations. Even where effective balancing can be achieved, it requires complex precautions and costly equipment to achieve the required precision.
A further limitation of conventional high resolution scanners is the precision of the drive system itself and the control system at nanometer resolution. Even the most precise actuators available used under rigorous closed-loop control cannot reliably and stably control a scanning head at nanometer resolution. The problems of precision and stability are further exacerbated as the scanning speed requirements increase. The result is typically that the control system itself introduces vibration to the scanning system.
There is therefore a need for a high-resolution scanning system which would eliminate or cancel vibration without transferring forces via a base, thereby reliably eliminating substantially all vibration from the base, and which would achieve reliable nanometer-order resolution of the scanning head without closed-loop feedback position control.